It is well known to utilize a ventilator, anesthesia machine, or pressure support device to deliver a fluid, such as oxygen, air, or other breathing gas or gas mixture, to an airway of patient to augment, supplement, or substitute the patient's own ventilatory effort and/or to treat the patient with a pressure support therapy. Of importance in such situations is the ability to accurately regulate or control the pressure, flow, and/or volume of gas delivered to the patient during the inspiratory phase of the respiratory cycle. For present purposes, the term “ventilator” is used to describe any system or device that delivers a flow of gas or pressurized gas to the airway of a user.
FIG. 1 illustrates the inspiratory components of a conventional ventilator. These components include a source of a first gas 30, such as air, and a source of a second gas 32, such as oxygen. The source of first gas typically includes a pressurized storage tank, blower, bellows, impeller, fan, piston, pressure generator, or the like, that provides pressured air at a pressure above ambient pressure. The source of oxygen is typically a pressurized oxygen storage tank, a central wall supply (typically found in a hospital), or an oxygen concentrator. In short, the sources of the first and second gas can be pressure generators that operate under the control of the ventilator, an independent gas supply, such as that available through a hospital's central gas delivery system, or a combination thereof.
In the embodiment of FIG. 1, a first valve 34 controls the mixing of the first gas and the second gas, and a second valve 36 control the pressure and/or flow of the gas mixture provided to the patient, as indicated by arrow A. Valves 34 and 36 are typically proportional valves, which are commercially available in a number of different variants. Such valves usually comprise an electromagnet, a membrane that usually is made of rubber, and a valve seat. The amount or percentage that the valve is opened is determined by the current flowing through the electromagnet. The resulting gas flow is approximately proportional to the current.
An alternative conventional inspiratory portion of a ventilator is shown in FIG. 2. In this embodiment, separate valves 38 and 40 control the supply of gas to the first gas (e.g., air) and the supply of the second gas (e.g., oxygen). The separate gas supplies are mixed downstream of the valves, typically using a mixing element or accumulator, for subsequent delivery to the patient. In each embodiment, the combined gas flow is carried by a primary conduit 42 to an external coupling provided on the ventilator housing. A flexible hose or patient circuit (not shown) couples to the external coupling an airway of the patient.
A conventional ventilator typically includes a flow sensor 44 and a pressure sensor 46 to measure the flow and pressure, respectively, of the gas delivered to the patient via the patient circuit. The output of flow sensor 44 and pressure sensor 46 is provided to a controller 50, which, in some ventilation modes, uses this information to control the flow, volume, and/or pressure of gas delivered to the patient. For example, the processor uses this information to control the actuation of valves 34, 36, 38, or 40 so that the desired flow, pressure, or volume of gas is administered to the patient.
In order to control the flow, volume, or pressure of gas delivered to the patient, the flow sensor and/or pressure sensors are used in a “closed loop” or feedback configuration. That is, the signals output by these sensors are provided to controller 50, which uses them to compare the actual flow, pressure, or volume, as determined from the sensors, with a set or desired quantity. The controller then controls the valves to reduce the error between the measured and the desired values. A typically controller 50 includes a PI or PID controller for comparing the measured values to the desired values and controlling the valves based thereon. An example of conventional ventilators that use such control techniques to provide pressure or volume control are described in U.S. Pat. Nos. 5,400,777 and 5,265,594. One disadvantage of conventional pressure/flow/volume control techniques, which is discussed in greater detail below, is that it is difficult for provide a control system that can quickly and accurately regulate the pressure, flow, or volume of gas using such techniques with a high level of stability.